1. Field of the Invention
The present invention relates generally to a method and composition for recovering metal values from flue dust and has particular utility for the steel industry where the control of furnace emissions is necessary.
2. Description of the Prior Art
In the production of certain grades of steel in this country, an Electric Arc Furnace (EAF) is used. The EAF process will be familiar to those skilled in the art. In a typical operation, solid charge ingredients including raw scrap, limestone, burnt lime, iron ore and ferro alloy additives are charged to a furnace. Furnaces in common use include the so-called "top charge" units. Such furnaces are equipped with a roof swing which permits the roof to swing aside when cold scrap is charged; a rocker/rail tilting arrangement which allows the furnace to tilt forward for tapping and backward for slagging; means for supplying additions through the furnace roof; and evacuation means for removing dust generated during the steel making procedure.
Electrodes are typically supported overhead and project downwardly through the furnace roof. The electrodes produce an electric arc which surges between the electrodes and scrap and produces heat which melts the charge and refines the steel. The molten steel is tapped at about 3,000.degree. F. into a ladle and is cast into blooms or poured into ingot molds.
Particulate emissions are generated at several points during the process. For instance, such emissions are generated during charging of the scrap, during tapping of the furnace, during pneumatic injection of additives, during oxygen blowing, as well as during melt down periods. Customarily in the industry, the EAF dust is collected in baghouses. Recent EPA rulings have determined that such inorganic dust constitutes a hazardous waste. As a result, the disposal of such collected dust presents an ever increasing problem and it has become mandatory to find suitable environmentally accepted methods of dust disposal.
One approach to the present problem is to reutilize the metal values present in the dust, such as zinc, lead, cadmium and chromium, to increase the concentration of such metal values to the point where it is economical to extract the metal values. Thus, repeated recycling of the arc dust results in a buildup of zinc, lead, cadmium and chromium concentrations in subsequently generated arc dust so that the concentrate may be economically recovered and sold, for example, in ingot form. Such a process substantially reduces the need for transfer of the arc dust to a certified landfill which must be monitored for leachate concentrations meeting EPA standards.
In order to successfully recycle the EAF dust to the furnace, it is generally necessary to bind, agglomerate or encapsulate the dust in some manner. Direct re-injection of the untreated dust tends to further pollute the work place. Certain of the prior art processes have attempted to pelletize the dust in order to enhance its storage and handling characteristics. For example, U.S. Pat. No. 4,407,672, issued Oct. 4, 1983, to Deuschle et al., contains a discussion of the production of "green ball" pellets made from EAF dust. Although such pellets have been successfully utilized in recycling processes of the type described, they typically involve a chemical bonding or agglomerating reaction which requires a predetermined cure time. Also, the previously known pellets have lacked the requisite degree of mechanical strength and, as a result, have been difficult to store or have partially disintegrated during transit and reinjection into the furnace.
A need thus exists for an improved method and composition for recycling metal containing EAF dust to produce a product having a saleable value while sparing the environment of a hazardous waste which must be confined at great expense or treated by other acceptable means of rendering the waste non-toxic.
A need also exists for such a process in which waste dust is pelletized in order to be recycled to the EAF furnace for concentration to a level which makes metal extraction more economical.
A need also exists for such a process and composition which does not require a cure time or a calcining step and which produces a product with improved mechanical strength over processes currently available which involve the incorporation of lime into pellets.